Future thermal environment in Japan using large ensemble climate dataset d4PDF—Can we hold outdoor activities at global 4 °C warming?

Abstract Recent extreme heat events are threatening outdoor human activities, and a robust understanding of the future thermal environment at the city-scale is becoming increasingly essential as global warming progresses. This study analyzed bias-corrected wet-bulb globe temperature (WBGT) values derived from a 5 km high-resolution large ensemble climate dataset, enabling robust estimation of extreme events without the need for extreme value theory, and investigated future extreme thermal environments in Japan. Under historical conditions (Hist) (1981–2010), the monthly mean daily maximum WBGT ( Ave ) exceeded 28 °C in the southern part of Japan, which is in the temperate zone. In the simulation of the globally 4 K-warmer future climate relative to the preindustrial period (GW4K), the thermal environment becomes increasingly severe: Ave exceeded 33 °C, which is the threshold indicating high heatstroke risk. Under GW4K, the thermal environment in the subarctic zone was comparable to that of the temperate zone under Hist. In August under GW4K, more than half of the days were classified as having a thermal environment where heavy exercise was prohibited, even at night. Consequently, engaging in outdoor activities will be risky in the future climate. Similarly severe conditions were observed in July and September. Moreover, the thermal environment in June under GW4K was comparable to that in August under Hist. The findings provide valuable insights into the need for public heat stress measures and personal adaptation to future global warming.